Add triangle edge guardband for Tex2D lookup diff

Add triangle edge guardband for Tex2D lookup diff

Previously 3D and cube textures allowed pixels along the seam of
triangles to use data from either triangle. This change copies that
allowance to the 2D texture computeTextureLookupDiff. The projection
values chosen for the mipmap.2d.projected group make the data
discontinous over the edge, making it sensitive to tie-breaker
resolution. This change resolves the issue.

Affects: framework

Test: Executed mipmap.2d.projected group with failing result images
Bug: 63335787
Change-Id: Ibaed590fb5e1bc9eb837c9027ddadcea2081f629